Production of metal oxide nanoparticles dispersed on fibres

ABSTRACT

The present invention relates to a method for obtaining nanoparticles of metal oxides dispersed in fibers comprising preparing a stable gel from nanoparticle precursors, immersing the fibers in the stable gel and subjecting them to a hydrothermal treatment until the fibers are coated with the nanoparticles in a homogeneous way. The obtained fibers are used in the degradation of organic pollutants.

TECHNICAL FIELD OF THE INVENTION

This invention is related to the industrial sector of materials, particularly with the methods to form hybrid systems of the textile industry that incorporate nanotechnology in polymeric fibers or textiles forming functionalized fibers.

BACKGROUND OF THE INVENTION

The addition of nanoparticles on the textile fibers may have disadvantages in the dispersion since the addition and incorporation is critical and difficult to achieve. Taking into account the nanometric sizes of metal oxide particles, there is a limitation for their dispersion, since due to their size, they have a high surface energy and a high tendency to agglomeration.

One of the techniques most used to obtain a hybrid material, from a textile fiber with nanoparticles, consists of immersing the fibers in sol-gel solutions that contain the precursors of the metal oxides and generate the crystalline formation through a calcination process. In this treatment, the fibers suffer degradation phenomena, given their organic nature. Therefore, this type of solution does not solve the problem of dispersion of nanoparticles.

Other methods of manufacturing the hybrid materials by immersion include taking advantage of the exchange of the surface charges that some polymeric fibers have to promote the anchoring of the nanoparticles from a colloidal solution. These techniques have many stages of manufacture and the need to maintain strict control in each of them. In addition, the final hybrid fiber material may have different coating thicknesses of metal oxide nanoparticles, which may decrease the porosity between the fibers and increase the stiffness.

The scientific article by Chaorong et al. “Functionalization of Electrospun Nanofibers of Natural Cotton Cellulose by Cerium Dioxide Nanoparticles for Ultraviolet Protection” develops a process to form nanoparticles of metal oxides on fibers. This research proposes the surface modification of cellulose acetate nanofibers with CeO₂ nanoparticles through a hydrothermal treatment. However, the nanoparticles obtained are of sizes greater than 100 nm and very little homogeneous.

On the other hand, it is known that after the synthesis of inorganic nanoparticles it is necessary to perform calcining processes at high temperatures to obtain the crystalline properties that define the behavior of the material. Generally, the subsequent thermal treatment is carried out at temperatures that are around 300 and 800° C., temperatures in which a greater quantity of crystalline material is obtained, avoiding the formation of an amorphous structure. For example, the study by Mahshid S. et al. “Synthesis of TiO ₂ nanoparticles by hydrolysis and peptization of titanium isopropoxide solution” ([Periodic publication]//Semicond.Physics, Quantum, Electron.-2006-pp. 65-68) proposes a sol-gel synthesis and a heat treatment at different temperatures. However, the minimum temperatures at which crystallinity is acceptable are around 400 and 600° C. It results in a significantly aggressive process for the polymeric fibers, therefore, for the processing of this type of organic-inorganic hybrid material it is unlikely to use this methodology.

Other existing techniques for depositing nanoparticles on fibers include: manipulation of the fibers during and after immersion, multiple immersions, hydrothermal treatment or subsequent calcinations, which causes the polymer fibers to degrade.

From these difficulties, the present development reduces the amount of processes related to the immersion of the fibrous substrates in the nanoparticle solutions, promotes the crystallinity of the nanoparticles in a single step, without the need to calcinate, and does not noticeably affect the porosity between the fibers.

BRIEF DESCRIPTION OF THE INVENTION

The present invention refers to a manufacturing process of nanoparticles of metal oxides dispersed in a fibrous hybrid material (polymeric fibers). The process for obtaining the hybrid material, that is to say the dispersion of the metal oxide nanoparticles on the surface of the polymer fibers, is carried out through the combination of the sol-gel technique and a hydrothermal treatment in a single step.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 SEM micrograph of the detail of hybrid nanofibers obtained by post-functionalization of PAN nanofibers by sol-gel with TiO₂ compositions.

FIG. 2 SEM micrograph of the detail of the hybrid nanofibers obtained by the post-functionalization of PAN nanofibers by sol-gel with TiO₂—CeO₂ 5% compositions.

FIG. 3 TEM micrographs of the post-functionalized nanofibers with sol-gel with compositions: a) TiO₂, b) detail at higher resolution of the nanofibers with TiO₂ g) TiO₂—CeO₂ 5%, h) detail of nanofibers with TiO₂—CeO₂ 5%, i) diffraction pattern of rings of nanofibers with sol-gel of TiO₂—CeO₂ 5%.

FIG. 4 TEM micrograph of post-functionalized nanofibers by sol-gel with TiO₂—CeO₂ 5% composition.

FIG. 5 Difractograms of post-functionalized nanofibers by sol-gel with TiO₂ and in the range of TiO₂—CeO₂ compositions.

FIG. 6 Comparison of the photocatalytic degradation in the modified nanofibers with the metal oxides through the processing routes in which the modification occurs from the formation or spinning of the fiber.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a method for coating fibers with nanoparticles which comprises preparing a stable gel from nanoparticle precursors, immersing fibers in said stable gel and subjecting them to a hydrothermal treatment until the fibers are coated with the nanoparticles.

For purposes of the present invention, fibers are understood as any set of filaments or strands capable of being used to form yarns and fabrics. The fibers are select from the group of natural organic fibers, synthetic organic fibers and inorganic fibers and combinations thereof. Among the natural organic fibers are fibers of vegetable and animal origin, for example cotton, capoc, linen, jute, hemp, ramina, sisal, coconut fiber, pineapple, wool, silk, hair. Among the synthetic organic fibers are those of cellulose composition (cellulose), non-cellulosic polymers, such as protein, rubber, aliphatic polyamide, aromatic polyamide, polyester, polyacrylonitrile PAN, polyurethane, polyethylene or polypropylene, polyvinyl chloride, polyvinylidene chloride, phenol navolaca base, tetrafluoropolyethylene, among others. Among the inorganic fibers are metallic and non-metallic fibers, for example fiberglass, carbon fiber, PAN carbon fibers, metal, boron, silica, silica carbide and asbestos, among others.

The fibers can be nanofibers (NFs), microfibers or larger fibers. Nanofibers may have a size of lower than 1000 nanometers, lower than 500 nanometers, or lower than 100 nanometers, microfibers a size between 10.0 and 0.5 micrometers, and the largest fibers a size greater than 0.5 micrometers. The fibers can be obtained by interfacial polymerization, electrospinning, forcespinning or any other equivalent technique known to a person of ordinary skill in the art.

By the method disclosed in the present invention, the above described fibers are modified by post-functionalization of fibers using a process that combines a method for the preparation of a gel and a hydrothermal treatment. In the process, a polymer solution is modified by adding metal oxide precursors and subjecting the fibers to a hydrothermal treatment to promote the crystallinity of the nanoparticles. Among its objectives, the method disclosed herein has the modification of fibers that, once subjected to the process, they exhibit morphologies different from the initial ones, giving them improved properties in the absorption of radiation, UV protection, antibacterial action, self-cleaning and high photocatalytic activity. By the term “modify” it is understood to cover or cover partially or totally, upholstery, lining, etc.

The first step of this process consists in preparing a stable gel by any technique known to a person moderately skilled in the art. Among the techniques to obtain a stable gel is the sol-gel technique. For purposes of the present application, “stable gel” is understood as a homogeneous solution that does not vary in time. Not obtaining a stable gel could cause precipitation of the particles and therefore a fiber with the characteristics of the present invention would not be obtained.

The sol-gel technique is a technique used for the formation of nanoparticles, which comprises low process temperatures, and may also be easily modified according to the synthesis needs. Although the sol-gel process may seem quite simple, many variables influence the quality of the product. Among them there is the metal oxide precursor, the solvent used, the use of acidic or basic catalysts and complex agents.

The sol-gel chemistry involves the hydrolysis and condensation reactions of the metal alkoxide. In addition, sol-gel synthesis promotes small particle sizes and mesoporous structures.

The stable gel has a pH lower than 5, pH between 4 and 5 or pH between 3 and 4.5. The acidification of the stable gel can be carried out by the addition of acidic aqueous solutions or by any other technique known to a person of ordinary skill in the art, and aims to maintain the colloidal stability of the nanoparticles. For better results, the addition of acidic aqueous solutions is carried out dropwise. The stable gel should have a solids content between 0.01% and 5.0% w/w, between 0.01% and 2.00% w/w or between 0.08% and 1.2% w/w. Obtaining a gel with neutral pH can result in poor stability, while acidic pH values favor the stability of the gels and prevent the formation of precipitates.

It is important to clarify that, in the literature, the reports found with the modification of nanofibers by sol-gel immerse the nanofibers in the gel and then wash them with distilled water and subject them to temperature. In the preliminary tests of the present invention said procedures were performed with unfavorable results due to the fact that nanofibers covered in a uniform manner were not achieved, but the formation of agglomerates of important sizes in the nanofibers. The method employed in this invention is a new way for the modification of nanofibers through of the sun-gel technique, avoiding immersion and washing steps before the hydrothermal treatment.

The stable gel is made from nanoparticle precursors. For purposes of the present invention, the nanoparticle precursors are metal alkoxides or transition metal alkoxides Among the nanoparticle precursors which are used in the present invention are titanium isopropoxide (TTIP), tetraethyl orthosilicate (TEOS), titanium ethoxide Ti(OCH₂CH₃)₄, zinc acetate (Zn(CH₃COO)₂·2H₂O), titanium butoxide Ti(OCH₂CH₂CH₂CH₃)₄, titanium tetraisopropoxide Ti(OCH(CH₃)CH₂)₄, magnesium di (1-propoxide), aluminum tri(2-isopropoxide), and combinations of the above with transition metal salts such as zinc nitrate (Zn(NO₃)₂·6H₂O, cerium nitrate (Ce(NO₃)₂·6H₂O, silver nitrate (AgNO₃) and titanium chloride (TiCl₄), among others. Optionally, additives such as solvents, stabilizing agents and/or additives to control pH are added. Among the solvents are for example isopropanol, ethanol, deionized water and acetone, among others. Stabilizing agents are selected from acetylacetone, polyethylene glycol, propylene glycol, polyacrylamide, ethylene glycol, cetyl trimethylammon bromide, and hydroxymethylcellulose, among others. Among the possible additives to control the pH are nitric acid, acetic acid, hydrochloric acid, ammonia, ammonium hydroxide.

The nanoparticles (NPs) depend on the precursors used to prepare the stable gel. For purposes of the present invention, the nanoparticles are metal oxides (NPsOm) or metalloid oxides. Among the nanoparticles (metal oxides) with which the fibers can be modified are: Au₂O₃, Ag₂O₃, Ag₂O, BaO, CaO, CaO₂, Cu₂O, Cu₂O₂, CuO, CoO, CrO, Cr₂O₃, CrO₃, FeO, Fe₂O₃, HgO, KO₂, K₂O₂, MgO, MnO, Mn₂O₃, MnO₂, Mn₂O₇, Na₂O, Na₂O₂, NiO, Ni₂O₃, PbO, Li₂O, SnO, SnO₂, TiO, Ti₂O₃, TiO₂, SiO₂, SiO, ZnO, ZnO₂, B₂O₃, GeO₂, As₄O₆, Sb₂O₃, Sb₂O₅, TeO₂ and a combination thereof.

After immersing the fiber in the stable gel with the precursors, a hydrothermal treatment is carried out, which promotes the crystallinity of the nanoparticles. The hydrothermal treatment can occur in any equipment known by a person moderately skilled in the art who subjects the material (fiber) to temperatures between 110 and 210° C. The hydrothermal treatment can be carried out, for example, in an autoclave where the fibers are immersed in the stable gel. The hydrothermal treatment is carried out at a heating rate between 1° C./min and 15° C./min, between 2° C./min and 8° C./min, and/or between 5° C./min and 20° C./min, a temperature between 110° C. and 210° C., a temperature between 140° C. and 180° C., a temperature between 155° C. and 170° C., for a holding time between 2 and 48 hours, between 6 and 24 hours, between 10 hours and 15 hours. Subsequently, a cooling is carried out, which can be performed, for example, by natural convection. The fibers are then removed, washed with distilled water and dried until a fiber modified with nanoparticles is obtained.

“Hydrothermal treatment” means a process which involves a solvent and temperature, wherein the solvent is generally water, which can be mixed with alcohols or other types of solvents.

As observed in the SEM micrographs (FIG. 1 and FIG. 2), quite heterogeneous surfaces are observed due to the growth of the nanoparticles, which also shows how the nanofibers are covered in a uniform way and confirms that there is no clogging of the pores between fibers. This implies that the nucleation and growth of the nanoparticles occurs directly on the nanofibers.

The present invention can be used for photocatalytic degradation, which is one of the most widely-studied methods, due to the fact that it efficiently converts solar energy into effective chemical energy. Therefore, it is used to degrade hazardous organic materials in air and water, absorb heavy metals, break down bacteria and viruses, among others. Also the product of the present invention is used in filtration and purification, wherein it is necessary to have porous substrates with permeation properties and which at the same time degrade harmful substances.

The present invention provides a method for obtaining modified fibers with nanoparticles with high yields, a good use of raw material, low temperatures and short process times. Reducing the amount of processes related to the immersion of the fibrous substrates in the solutions, promoting the crystallinity of the metal oxide nanoparticles (NpsOm) in a single stage without the need to calcinate (process at low temperature) and without significantly affecting the porosity between the fibers.

It should be understood that the present invention is not limited to the embodiments described and illustrated, so as it will be evident to a person skilled in the art there are possible variations and modifications that do not depart from the spirit of the invention, which is only defined by the claims.

Example 1.

The PAN polymer is dissolved in the DMF solvent for a solution with a concentration of 8% w/v. This solution is electrospinned and the fibers obtained are kept apart for the hydrothermal treatment.

Example 2. Process for Obtaining Modified Nanofibers with TiO₂ Nanoparticles

a) Prepare a Stable Gel from Nanoparticle Precursors:

For the preparation of a stable gel TTIP (as a precursor of TiO₂) was added by mixing the TTIP with magnetic stirring in an isopropanol and acetylacetone solution, until a homogeneous solution was obtained. After homogenization, this solution was added dropwise in water with addition of glacial acetic acid (pH 2) and kept in vigorous agitation for 2 hours. By proceeding with the following molar ratio:

Reagent TTIP Acac IsoOH Water Molar ratio 1 0.25 2.0 20.0

The stable gel obtained was diluted in water with addition of acids (pH 2) until obtaining a solids percentage of 0.1% A stable diluted gel is obtained.

b) Immerse Some Fibers in the Stable Gel and Subject them to a Hydrothermal Treatment until the Fibers are Coated with the Nanoparticles:

About 80 mg of polyacrylonitrile nanofibers (PAN) obtained by Example 1 were taken and immersed in 70 mL of the stable gel diluted in step (a). Subsequently, they are subjected to a hydrothermal treatment in an autoclave at 160° C. for 12 hours. It is suggested that the equipment be filled to a volume between 50% and 80%.

c) Washing

After the thermal treatment, the polyacrylonitrile nanofibers were removed from the autoclave and washed 5 times with distilled water until the excess of generated nanoparticles was removed.

With the proposed process, PAN nanofibers covered in the surface were obtained by TiO₂ nanoparticles, as shown in the micrograph of FIG. 1. This implies that the nucleation and growth of the nanoparticles occurs directly on the nanofibers.

Example 3. Process for Obtaining Modified Nanofibers with TiO₂ and CeO₂ Nanoparticles

A second precursor was added, this time a precursor of CeO₂ nanoparticles, wherein the nanoparticle precursor was Ce(NO₃)₃. 6H₂O obtaining the combination of TiO₂—CeO₂. For a 5% molar ratio of CeO₂, 133 mg of Ce(NO₃)₃. 6H₂O were placed to obtain 5 mL of stable gel. The micrograph obtained for this example is seen in FIG. 2.

Example 4. Other Nanofibers Modified with Nanoparticles

Solids Temperature Time Fiber Precursor NP content (° C.) (hours) Cotton (natural TTIP TiO₂ 0.1% 110 12 organic fiber) Cellulose TEOS SiO₂ 0.5% 200 10 Polypropylene Zinc acetate ZnO₂ 1.5% 250 8 (Zn(CH₃COO)₂•2H₂O) Glass TTIP and Ce(NO3)3•6H2O TiO₂—CeO₂ 4.0% 120 24 (inorganic fiber)

Example 5. Characterization of Crystalline Properties

Characterization of the crystalline properties of the samples disclosed in Examples 2 and 3 with different ratios of TiO₂ and CeO₂ was made. In the diffractograms (FIG. 5) diffraction peaks are observed related to the formation of crystalline nanoparticles in the composition range TiO₂ and CeO₂ tested. At 26° C. the characteristic peak of the anatase crystalline phase of TiO₂ is observed, which indicates that the process carried out at low temperatures ensuring the formation of TiO₂ crystals on the fibers.

Example 6. Characterization of Nanofibers by SEM Micrographs

Through the method of the present invention where, due to the post-functionalization of the PAN nanofibers with the TiO₂ sol-gel in the hydrothermal treatment of Example 2, hybrid nanofibers were obtained, as shown in the SEM FIG. 1.

The surface detail of the sol-gel modified nanofibers of Example 3 is seen in FIG. 2. The nanofibers are covered in a uniform way, which implies that the nucleation and growth of the nanoparticles occurs directly on the nanofibers.

Example 7. Characterization of the Nanofibers by TEM Microraphs and Diffractograms

The average diameter of the nanofibers obtained through post-functionalization with sol-gel is not expected to change significantly with respect to the initial value of the fibers. The results of crystallinity of the fibers obtained by Example 2 are seen in FIG. 3a and FIG. 3b . The hydrothermal treatment is favorable to promote the crystallinity of the TiO₂—CeO₂ 5% nanoparticles obtained by Example 3. This is seen in the ring diffraction pattern of FIG. 3i , which are related to the crystallinity of the polymer nanofibers by the plane (0 0 2), and planes (1 0 1), (0 0 4), (2 0 0) and (1 0 2) of the anatase crystal phase. In the same way, this is confirmed by the X-ray diffractograms obtained for the samples in FIG. 5.

Example 8. Degradation Efficiency Measures

The degradation efficiency evaluation of methylene blue as a model substance for the simulation of an organic pollutant in the presence of solar radiation using the modified nanofibers obtained by means of Examples 2 and 3 was carried out.

The comparison of the degradation efficiencies of methylene blue are observed in FIG. 6, wherein they are compared to two other nanoparticle incorporation processes in nanofiber in which the modification occurs from the formation or spinning of the fiber. When the nanoparticles are incorporated into the nanofibers in an immersion process (NFs+NPs), the lowest efficiencies are obtained. When the nanofibers are immersed in the precursors (NFs+precursors), the efficiency is increased and finally, when carried out by the process of the present invention, the efficiency is much more increased. In conclusion, the samples obtained with the process proposed in the present invention had higher yields than those exhibited by the other processes tested. 

1. A method for modifying fibers with nanoparticles, the method comprising: preparing a stable gel from nanoparticle precursors; immersing one or more fibers in the stable gel; and subjecting the one or more fibers to a hydrothermal treatment until the one or more fibers are coated with the nanoparticles.
 2. The method according to claim 1, wherein the stable gel is prepared by a sol-gel technique.
 3. The method according to claim 1, wherein the stable gel has a pH lower than 5 and a solids content between 0.01 and 5.0%.
 4. The method according to claim 1, wherein the nanoparticles are metal oxides.
 5. The method according to claim 1, wherein the nanoparticle precursors are metal alkoxides.
 6. The method according to claim 1, wherein the one or more fibers are selected from the group of natural organic, synthetic organic and inorganic fibers and combinations thereof, in nano or micro scale.
 7. The method according to claim 1, wherein the hydrothermal treatment is carried out at a temperature between 100° C. and 300° C., and for a period of time between 6 and 24 hours.
 8. The method according to claim 1, further comprising washing the fibers coated with the nanoparticles. 